This invention relates to apparatus for the detection of nuclear magnetic resonance and more particularly to nuclear double resonance apparatus which enables one to use the strong resonance of one spin system to detect the weak resonance of a second spin system. The present invention also relates to other systems which can be mathematically described in a pseudospin formalism, that is, systems whose mathematical equations of motion can be cast in the same form as equations of motion for magnetic resonance and for which analogous double resonance is possible.
Nuclear magnetic double resonance spectrometers are well known to those skilled in the art. Such apparatus, for example, is disclosed in a publication, "Nuclear Double Resonance in the Rotating Frame," S. R. Hartmann and E. L. Hahn, PHysical Review, Vol. 128, pp. 2042-2053, (1962), and in "Spin Temperature in Nuclear Double Resonance," F. M. Lurie and C. P. Slichter, Physical Review, Vol. 133, pp. A1108-A1122, (1964). Such spectrometers are a type of nuclear magnetic resonance (NMR) spectrometer in which two RF excitations are applied to a single material sample. The sample contains two nuclear species:D, which is indicative of an abundant species, and S, which is indicative of spins of a rare species. The two nuclear species of spin I.sub.D and I.sub.S have gyromagnetic ratios of .gamma..sub.D and .gamma..sub.S, respectively, and are acted on by RF magnetic fields 2H.sub.1D cos(2.pi. f.sub.D t) and 2H.sub.1S cos(2.pi.f.sub.S t), while being located in a fixed external magnetic field H.sub.0. When the frequencies f.sub.D and f.sub.S respectively designate the NMR or Larmor frequencies of the D and S spins, nuclear double resonance between the D and S spins occurs when the amplitudes of the two RF excitations are adjusted so they satisfy the condition: EQU .gamma..sub.D H.sub.1D = .gamma..sub.S H.sub.1S. (1)
when this condition is satisfied, the D and S spin systems in the static magnetic field H.sub.0 become strongly coupled, even though the respective Larmor frequencies are widely different. The coupling occurs by means of the dipolar interaction between the D and S spins. This may be explained by noting that the precession of the S spins about H.sub.1S in the S-spin rotating frame causes the component of the dipolar field along the direction of the static field H.sub.0 to oscillate at an angular frequency .gamma..sub.S H.sub.1S. When the condition of equation (1) is satisfied, this frequency of alternation is just such as to induce transitions of the D spins relative to the rotating field H.sub.1D. These transitions of the D spins are in essence "rotary saturation" transitions, which were first described in "Nuclear Magnetic Resonance Saturation and Rotary Saturation in Solids," A. G. Redfield, PHysical Review, Vol. 98, pp. 1787-1809, (1955). Relatively rapid phase changes of the alternating field H.sub.1S enable this coupling to produce transitions in the D system in a semi-continuous way. Thus, even though the resonance of the S spins may be difficult to observe directly, it can be observed indirectly by its effect on the D spins. Reference to the Hartmann and Hahn and Lurie and Slichter publications is suggested for a more exhaustive explanation of this phenomenon.
Because the interaction between the S and D spin systems is essentially a noiseless channel, the effective signal-to-noise ratio enhancement of the S spin system is proportional to the time of the measurement. In conventional detection, on the other hand, the signal-to-noise ratio enhancement is proportional only to the square root of the time of the measurement. Double resonance techniques, therefore, greatly increase the sensitivity of NMR spectroscopy.
A variation of the double resonance technique has, in addition, been described in a publication by A. Pines et al. entitled "Proton-Enhanced NMR of Dilute Spins in Solids," which appeared in the Journal of Chemical Physics, Vol. 59, pp. 569-590, (1973). In this variation, the D spins are made to interact with the S spins, again through satisfying equation (1), this time with the D spins being used to polarize the S spins, whereupon the NMR signal of the S spins is directly detected. The signal-to-noise ratio enhancement in this instance occurs as a result of the added polarization of the S spins. This concept, moreover, is additionally set forth in U.S. Pat. No. 3,792,346 entitled "Proton-Enhanced Nuclear Induction Spectroscopy," M. G. Gibby, et al.
While dual sample systems have been utilized heretofore see, for example, the publication entitled "EPR Study of Tetracene Positive Ion," J. S. Hyde and H. W. Brown, Journal of Chemical Physics, Vol. 37, pp. 368-378, (1962) at page 371 and U.S. Pat. No. 3,487,293, T. Seki, et al. entitled "Method of Field/Frequency Control During Sample Exchanges," double resonance methods have heretofore been accomplished with spectrometers which are designed to operate on the assumption that the S and D spins are in the same piece of material.